The invention relates to multimedia signal processing, and in particular relates to image watermarking methods and related applications.
Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee""s co-pending application Ser. No. 09/503,881 and U.S. Pat. No. 5,862,260, which are hereby incorporated by reference.
The invention provides halftone image watermark methods and systems. One aspect of the invention is a method of halftone image watermarking. This method assigns a set of halftone watermark values to locations within a halftone image. It diffuses error associated with the halftone watermark values to neighboring locations within the halftone image. The error is characterized as a difference between a multilevel pixel value at a location of a halftone watermark dot, and the halftone watermark value of the halftone watermark dot. This method may be used in conjunction with other watermark embedding stages. For example, a robust watermark carrying a key for the halftone watermark may be embedded in the image before embedding the halftone watermark.
Another aspect of the invention is a method of decoding a halftone watermark from an image. The decoding method uses a key to identify locations and values of a halftone watermark, and analyzes pixel values at the locations to determine whether the values at the locations correspond to the values specified by the key. In one application, a robust watermark embedded in the image carries the key used to decode the halftone watermark from the image. A decoder reads the robust watermark from a scan of the halftone watermarked image, optionally compensating for geometric distortion of the scanned image using an orientation signal. The decoder then extracts the key from the robust watermark and passes it to a verifier, which in turn, examines a high resolution scan of the suspect image to determine whether the halftone watermark is present at locations specified by the key.
Another aspect of the invention is another method of embedding a watermark in a halftone image. This method computes a watermark image comprising an array of values corresponding to pixel locations in a halftone image. It embeds the watermark image in the halftone image by using the values of the watermark image to modulate thresholds at the pixel locations. The thresholds are used in a halftone process to convert multilevel pixel values in a multilevel per pixel image into halftone pixel values of the halftone image.
Another aspect of the invention is yet another method of embedding a watermark in a halftone image. This method computes a watermark image comprising an array of values corresponding to pixel locations in a target halftone image at a halftone resolution. It then combines the array of values of the watermark image with corresponding multilevel pixel values of a multilevel per pixel image to create a watermarked multilevel per pixel image at the resolution of the target halftone image. Finally, it performs a halftone process to convert the watermarked multilevel per pixel image to a watermarked halftone image.
Finally, another aspect of the invention is a watermark decoder. The decoder includes a watermark detector and reader. The detector analyzes portions of an image to detect a watermark signal embedded in the image. The image is scanned from a halftone printed image at a sufficiently high resolution to discern a watermark image embedded at a resolution of the halftone image. The watermark reader reads a watermark signal from the portions of the image and decodes an auxiliary message comprising one or more symbols.